(1) Field of the Invention
The present invention relates to a method of minimizing the attitude hump phenomenon and to a rotary wing aircraft provided with means for performing the method and in particular a helicopter.
(2) Description of Related Art
Conventionally, a helicopter comprises an airframe extending longitudinally from a front end to a rear end on either side of an antero-posterior plane of symmetry, and in a vertical direction from a bottom portion fitted with landing gear towards a top portion fitted with a main lift and propulsion rotor.
Furthermore, the airframe has a tail rotor at its rear end. The tail rotor serves to counter the torque exerted by the main rotor on the airframe, and it also enables the pilot to control yaw movements of the helicopter.
Furthermore, a helicopter sometimes has additional stabilizer surfaces. For example, it is common practice to fit a helicopter with a yaw stabilizer surface, e.g. contained in the antero-posterior plane in order to combat said torque at high speed.
Similarly, a helicopter sometimes has pitching-stabilizer means presenting an angle having an absolute value lying in the range 0° to 90° relative to said antero-posterior plane, said pitching-stabilizer means comprising, for example, two pitching-stabilizer surfaces extending symmetrically on either side of said antero-posterior plane.
Such pitching-stabilizer means are sometimes referred to as a “horizontal stabilizer”.
The pitching-stabilizer means are very effective, with the effectiveness thereof increasing with increasing forward speed of the helicopter. Furthermore, it can be understood that the effectiveness of pitching-stabilizer means is maximized by maximizing its wing area.
However, on a traditional helicopter flying in translation at low speed or even hovering, the air that passes through the main rotor is deflected rearwards and impacts against the pitching-stabilizer means. This air passing through the rotor then exerts on the pitching-stabilizer means forces that the pilot needs to compensate by operating the flight controls.
Nevertheless, when flying conditions vary, the deflection of said air is also modified. The same applies to the forces exerted on the pitching-stabilizer means.
This phenomenon is referred to as an “attitude hump” by the person skilled in the art.
While accelerating or decelerating, the attitude hump, i.e. the forces exerted by the air passing through the main rotor and impacting against the pitching-stabilizer means, tends to give the helicopter a nose-up attitude. In order to stabilize the speed of the helicopter, the pilot then needs to use the control stick for controlling the cyclic pitch of the blades of the main rotor so as to reduce the nose-up attitude of the helicopter.
As the forward speed of the helicopter increases, the air stream coming from the main rotor becomes deflected towards the forward axis and therefore passes over the pitching-stabilizer means and no longer gives rise to the attitude hump phenomenon.
It can be understood that optimizing the pitching-stabilizer means by maximizing its wing area accentuates the attitude hump.
Under such circumstances, it would appear to be impossible to use pitching-stabilizer means having a large wing area on a helicopter, without that giving rise to an increase in the attitude hump phenomenon.
It should be observed that the technological background includes Document EP 1 547 919 relating to a helicopter provided with pitching-stabilizer means. Those stabilizer means may comprise two pitching-stabilizer surfaces extending symmetrically on either side of said antero-posterior plane.
The stabilizer surfaces may be horizontal by being orthogonal to the antero-posterior plane, for example, or they may together describe a V-shape, by presenting an angle lying in the range 0° to 90° relative to said antero-posterior plane.
Each pitching-stabilizer surface may be fitted with a flap.
Thus, EP 0 229 896 describes a rotary wing aircraft having upper pitching-stabilizer means vertically above lower pitching-stabilizer means. Such a device is sometimes referred to as a “multi-plane stabilizer”.
Documents U.S. Pat. No. 2,369,652 and GB 606420 relate to a rotary wing aircraft having upper pitching-stabilizer means and lower pitching-stabilizer means that are slightly offset relative to a longitudinal direction of the aircraft.
Furthermore, the teaching of documents U.S. Pat. No. 3,583,659 and NL 0 229 896 is also known.
Those documents provide teaching that is of interest, but it nevertheless remains remote from minimizing the attitude hump phenomenon.